Sea slug amputates tip of its penis

After this sea slugs has sex, its penis drops off. Fortunately, it has a spare.

Like most sea slugs, Chromodoris reticulata are simultaneous hermaphrodites: they have both male and female sex organs. When two mate, each extrudes a penis into the other's vagina and transfers sperm (see photo below; the thin white organs can be seen stretching between the sea slugs).

(Image: Ayami Sekizawa et al/Royal Society)

Ayami Sekizawa of Osaka City University in Japan and colleagues watched the sea slugs after mating, and saw that their penises dropped off within 20 minutes. Or rather, part of them did: each penis was made up of three segments, and only the top segment was used for any one mating. This could be a tactic to reduce the chance that their partner will be fertilised by another slug's sperm.

It then took 24 hours to get the next segment into working order, ready for another go.